1. Field of the Invention
The present invention relates to an X-ray fluorescence spectrometer for analyzing the coating weight of and the composition of a coating of a galvanneal sheet steel.
2. Description of the Conventional Art
Hitherto, the technique is well known in the art, of analyzing the coating weight of and the composition of a coating of a galvanneal sheet steel through the X-ray fluorescence analysis. (See, for example, the Japanese Laid-open Patent Publications No. 58-223047, No.55-24680, No. 60-236052, No. 2-257045, and No. 4-232448.) According to those known techniques, based on the idea discussed below, the intensities of the fluorescent X-rays are measured with the use of two combinations of the angle of incidence of primary X-rays and the angle of detection of fluorescent X-rays, in which combinations the angles of incidence and/or the angles of detection are different from each other.
In general, when the coating weight of a target film to be measured is increased, the intensity of the fluorescent X-rays then obtained increases as well, but there is the uppermost limit which is referred to as the infinite thickness intensity. With respect to the target film to be measured having a certain composition, in the measurement system including an X-ray source, of which primary X-rays have a predetermined wavelength distribution, and a detecting device for detecting fluorescent X-rays of a predetermined wavelength, once the angle of incidence of the primary X-rays and the angle of detection of the fluorescent X-rays are fixed, the depth to which such measurement system can measure the target film to be measured is fixed. This measurement depth is indicated by the coating weight, at which the intensity of the fluorescent X-rays attains a value equal to 99% of the uppermost limit referred to above, that is, the infinite thickness intensity.
In view of the above, using the two measurement systems, of which measurement depths are different from each other, the coating weight of and the composition of the coating are analyzed by obtaining different information (the different intensities of the fluorescent X-rays) from different depths of the coating, but according to the conventional techniques, the angle of incidence of the primary X-rays and the angle of detection of the fluorescent X-rays are so set that with respect to the measurement system of which measurement depth is shallow, the measurement depth can be smaller than the coating weight of the coating, whereby the composition of the coating is determined by removing information from the substrate sheet steel and relying only on the intensity of the fluorescent X-rays obtained with the measurement system, of which measurement depth is shallow.
However, since the intensity of the fluorescent X-rays given by the measurement system is low as the position from which the fluorescent X-rays are emitted is deep, that is, since the measurement sensitivity of the measurement system is low with increase of the depth of the target site, the measurement system, of which the measurement depth is shallow, according to the conventional art lacks a measurement sensitivity at the substrate sheet steel and the deep position in the coating from the surface and a large difference occurs between the measurement sensitivity at a shallow (adjacent the surface) position of the coating and the measurement sensitivity at a deep position (adjacent the substrate sheet steel) of the coating. Thus, since according to the conventional techniques, the composition of the coating is determined almost based on only the intensity of the fluorescent X-rays from the shallow area of the coating, an accurate analysis is possible with, for example, an electrogalvanized Zn—Fe alloy sheet steel having a coating of a composition uniform in a direction of depth thereof, but sufficiently accurate analysis is impossible with the galvanneal sheet steel having a coating of an inhomogeneous composition in a direction of depth thereof.